Compressive strength, split tensile strength, ultimate tensile strain and axial tensile strength of concrete with 0%, 5%, 8% and 12% light burnt MgO-based expansive agent were tested. By means of scanning electron microscope and optical microscope, the microstnicture of concretes was examined. The results indicate that restraint may improve mechanical properties of concrete to some extent, and three-dimensionaliy restrained concrete shows best mechanical properties when concrete contains the same amount of MgO. Compressive strength of three-dimensionally restrained specimens with 5%, 8% and 12% MgO increases by 16.9%, 22.0% and 16.0% respectively at 360 d, and splitting tensile strength increases by 32.93%, 31.03% and 33.33% respectively at 180 d compared with unrestrained specimens. Ultimate tensile strain of three-dimensionally restrained specimens with 5% and 8% MgO increases by 9.5% and 3.1% respectively, and tensile strength increases by 5.7% and 6.9% respectively compared with unrestrained specimens. Under the free-curing condition, microstnicture of expansive cement paste and concrete is loose, with cracks in the paste and matrix-aggregate interface. Restraint could improve the microstnicture of expansive concrete with light burnt MgO, especially by densifying and intensifying the matrix-aggregate interface. As a result, the mechanical properties of concrete are improved.%研究掺0％,5％,8％和12％(质量分数,下同)轻烧MgO混凝土的抗压强度、劈裂抗拉强度、极限拉伸值和轴拉强度随龄期的变化规律,并用扫描电子显微镜和光学显微镜对混凝土微观结构进行分析.结果表明:掺轻烧MgO混凝土在不同约束条件下力学性能都有不同程度的提高,3维约束对混凝土力学性能提高程度最明显.掺5％,8％和12％ MgO混凝土3维约束试件360 d时抗压强度分别比自由试件高16.9％,22.0％和16.0％,180 d时劈拉强度分别比自由试件高32.93％,31.03％和33.33％.掺5％和8％ MgO混凝土3维约束试件极限拉伸值比自由试件分别提高9.5％和3.1％,抗拉强度分别提高5.7％和6.9％.自由条件下,MgO掺量较大时,MgO水化膨胀使得混凝土内部结构松散,基体呈疏松、多孔状,骨料与界面黏接较弱；约束条件下,混凝土基体的致密性得到提高,基体和集料界面的黏接能力加强,从而提高了混凝土的力学性能.
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